This invention relates generally to medical imaging systems, and more particularly, to a phased array coil for a Magnetic Resonance Imaging (MRI) system.
MRI or Nuclear Magnetic Resonance (NMR) imaging generally provides for the spatial discrimination of resonant interactions between Radio Frequency (RF) waves and nuclei in a magnetic field. NMR signals are typically detected using a receive RF coil, which is placed in the vicinity of a volume excited by the magnet system of the MRI or NMR scanner.
It is known to use a phased array configuration for RF coils in MRI to receive NMR signals. For example, a phased array surface coil for horizontal field MRI systems may include a surface phased array having multiple loop element coils. Any two adjacent loop elements typically have physical overlap in order to provide isolation. The multiple loop elements can be arranged, for example, in one row or in a triangle shape array.
Typically, a set of surface coil loops are critically decoupled relative to each other and simultaneously receive the NMR signal that is generated in the imaging region that is excited by a dedicated volume coil. The signal from these surface coils is then reconstructed to generate an image that is multiple times larger than the size of each individual loop in the array. Because the size of each element in the phased array is relatively small compared with the final reconstructed image, the noise figure is reduced and the signal obtained from each element of the phased array is increased. Thus, the final result is a significant increase in SNR for the final reconstructed image. The first application of such a phased array design was intended for the spine region because the required coverage of the spine area for the human body is extremely large and a single receive coil structure could not adequately provide the image necessary for clinical evaluation.
The use of these phased array coil structures, for example, a row of square loop coils for spine scanning when the coils are lined up in the head-foot (HF) direction, results in a shading problem with axial and coronal imaging. When a loop coil is used to scan the human body in a 3 T or higher field strength system, the sensitivity map of the coil in the transverse plane that is perpendicular to the main field direction of the system is not symmetrical. If the main field directs from the back of magnet to the front (patient end), the sensitivity map of the loop coil, which is put under the human body, tilts to left side (on the transverse plane, not the left side of the human body). If the main field direction reverses, the sensitivity map tilts to right side. Thus, if the left-right center of this phased array is positioned to the center of the spine, the coil does not provide the best SNR in the spine region because the coil sensitivity tilts to one side. The signals received from the right and left side of the spine are not symmetrically equal. The intensity difference between the signals from two symmetrical point on the left and right side of the spine can be more than twenty percent. This causes the transverse and coronal images of the coil to have obvious left-right shading.
Other variations use a two-dimension phased array surface coil structure having circular-shaped loop coils positioned adjacent loop coils at three apexes of an equilateral triangle. This structure is not suitable for spine imaging. When a phased array coil is used for spine imaging it should be able to scan any section of the spine. The coil should be made of several coil stations that are lined up in the HF direction. This triangle shape array is not suitable to make a spine coil with several coil sections in the HF direction.
Various modifications and deviations to the phased array coil were developed. For example, a phased array coil with figure “8” shaped element coils was developed and are which are commonly known as “butterflies.” Instead of using a loop coil array, a quadrature coil array for spine imaging has also been provided. This coil is made of four quadrature pairs lined up in the HF direction with each quadrature pair having a loop and a butterfly element coil. This coil has several sections in the HF direction. Each section is made of one loop coil and a butterfly coil and they are quadrature analogically combined inside the coil or inside the MRI system. When a spine coil with this structure is used in 3 T system, the images generated by the coil also have left-right (LR) shading problems. This is because not only the loop coil, but also that butterfly coil has a tilted asymmetrical sensitivity map when used in a MRI system with 3T or higher main field. The sensitivity map of the butterfly coil is shifted to the same direction as the loop coil because the field inside the human body generated by the two wings of the butterfly coil are not symmetrical. The sensitivity map looks like two mountain peaks and the peak on the side to which the field tilts is higher than the other side. Thus, the signals received by the loop and butterfly coil quadrature analogically combined field still show an asymmetrical pattern. The best SNR area of this coil is not in the center where the spine lies, but shifts to one side. For example, one of the images on one side of the kidney is brighter than the other side.
Other coil arrays are also known that include six quadrature pairs. The six pairs are aligned in the HF direction to cover the whole Cervical-Thoracic-Lumbar (CTL) spine area. The first pair may be positioned on a former that is conformal to the human neck. The quadrature pairs can be used separately or jointly to image any desirable section of the spine. Two elements in each pair are either quadrature analogically combined or digitally combined depending on how many receive channels are in the MRI system.
In order to enhance the ability to image the CTL spine area, a phased array design was proposed that covered the intended area. The proposed design included six quadrature pairs (six loop and six saddle coils in a quadrature configuration) separated in a multiple list of modes in which the coil can operate and further can image any desirable section of the spine. These coils have six quadrature pairs lined up in HF direction. The first pair that covers the neck is put on the former with a conformal shape to the neck. The butterfly coil of this quadrature pair looks more like a saddle coil. All the quadrature pairs in coils are either analogically combined inside the coil or digitally combined in the system. The sensitivity map of a saddle coil also has an asymmetrical pattern. Because both two element coils of a quadrature pair have asymmetrical sensitivity, the digitally combined images still have the LR shading problem. Thus, when a coil with this structure is used in a system with 3 T or higher main field, it still has the left and right shading problem no matter whether it is analogically combined or digitally combined. An alternative embodiment is to use twelve elements independently or in any receiver configuration based on the MRI system. This system still has the LR shading problem.
Thus, in operation, when a surface coil (e.g., a loop coil) is used in high field MRI systems whose main magnetic field strength is 1.5 Tesla (T) or higher, an asymmetrical signal sensitivity of the coil is observed. Even though the loop coil has a symmetrical structure, the sensitivity map of the coil tilts to one side. The tilting direction is related to the direction of the main magnetic field. This phenomenon is more obvious in a system that has the main magnetic field strength equal or higher than 3 T. This phenomenon is caused by the dielectric and conductive effect of the human body on the high RF frequency electromagnetic field. The RF electromagnetic field generated by nuclei resonant spins inside the body is a circular polarized field and propagates inside the body because the electromagnetic wavelength inside the body is not much larger than the dimension of the body at high RF frequency. The MR signals received by the coil from the left and the right side are no longer equal because of propagation of the circular polarized field generated by nuclei resonant spins.
When a traditional spine coil (e.g., CTL coil) with the quadrature structure described above is used in a 3 T system, the coil sensitivity map is tilted to one side. For example, on transverse and coronal spine images, the signals of tissues on the left and right sides of the spine are not equal. The difference of the signal intensity on the two sides can cause LR shading on the transverse and coronal spine images. This is known as LR shading problem for spine imaging with the traditional CTL coil. This problem may result in an improper or failed diagnosis.
Further developments in MRI include Simultaneous Acquisition of Spatial Harmonics (SMASH) that use a parallel processing algorithm to exploit spatial information inherent in a surface coil array. The result is an increase in MR image acquisition speed, resolution and/or field of view. In a similar fashion, another parallel processing algorithm is known where the acceleration of image acquisition is performed on the time domain space instead of the frequency domain space. This parallel acquisition technique is referred to as Sensitivity Encoding (SENSE). In SENSE, images are obtained by means of magnetic resonance (MR) of an object placed in a static magnetic field and includes simultaneous measurement of a number of sets of MR signals by application gradients and an array of receiver coils. Reconstruction may bee provided from a number of receiver coil images from the sets of MR signals measured and/or reconstruction of a final image may be from a distant dependent sensitivity of the receiver coils and a first plurality of receiver coil images.
The characteristics of all of these parallel imaging techniques is that the acceleration speed is directly proportional to the number of independent receivers along the direction that the image acceleration needs to be applied. Thus, the higher the number of receiver coils, the faster the acceleration speed for acquiring an image with better SNR and improved image quality.
However, if the phased array surface coil has one row of square loop coils, it only has parallel imaging capability in the HF direction. Further, the quadrature phased arrays described above also only have parallel imaging capability in the HF direction. The two element coils of a quadrature pair in these coils have a large common coverage in the LR direction. The g-factor values for such a quadrature coil are severely elevated because the B1 field profile for these two element coils is nearly indistinguishable around the spine region. Thus, they have no parallel imaging capability in the LF direction.